Passive type detection systems are known and widely used. The passive type system is based on a phenomenon that a living thing radiates infrared having an intensity according to the body temperature. The known system is constructed to focus infrared radiating from a human passing through a predetermined detection region, and transmits a focused ray to an infrared detecting element whereby a change in the level of infrared energy from the detection region is converted into voltage so as to output a signal. If the signal is found to exceed a reference value, any form of alarm is given. Such detection systems are used not only as intrusion detection systems but also as switches at automatic doors to know in advance that a visiting guest has arrived.
A problem of the known detection system is that it is likely to produce an alarm owing to a sudden rise in the ambient temperature around the detection region caused by strong wind, microwave noise, sunlight, or any other interference. In order to prevent the production of false signal, an error preventive device is provided, which will be described by reference to FIG. 12:
A detector 1 is provided with a pair of infrared sensors 1a and 1b (three or more sensors can be used) which are arranged in parallel or in series with opposite polarity. An optical system 2 is located and detection regions E1 and E2 having a human height are set up.
When a human H or a dog M passes through the detection regions E1 and E2, it cannot instantly pass through the two regions. A time interval from the region E1 to the region E2 is unavoidable. This is a different point from ambient interference such as sunlight which covers the two regions E1 and E2 simultaneously. The outputs from the regions E1 and E2 due to ambient interference are mutually negated because of the differential electrical connection, thereby avoiding the production of false alarm. When a human intruder H passes through the detection regions E1 and E2, the human covers the whole space of each detection region E1 and E2, thereby outputting a signal at a level higher than the reference level. If a moving object is not a human but an animal such as a dog or a cat shorter than a human, it only covers a lower part of the detection regions E1 and E2, thereby outputting a signal at a lower level than the reference level. Thus the production of a false alarm is avoided.
When a difference between the temperature of a moving object and the ambient temperature is small, a false signalling can be avoided as shown in FIGS. 13 and 14. The signal output by a human H is higher than a reference level as shown in FIG. 13(a) whereas the signal output by a small animal M is lower than the reference level as shown in FIG. 14(a). When the difference is large, a false signal is likely to occur as shown in FIG. 13(b), because the signal output by a dog M exceeds the reference level. As is evident from FIGS. 13(b) and 14(b), it is difficult to ascertain whether the moving object is a human or an animal. If any object other than a human is detected and signalled, a fuss may occur.